<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<select id="sel">
    <option>北京</option>
</select>
<body>
            <script>
             let option= document.createElement("option")
             option.innerText='上海';
            let select= document.getElementById("sel");
            select.appendChild(option);
                let selectHtml = document.getElementById("sel").innerText;
                selectHtml +='<option>上海</option>'
                document.getElementById("sel").innerText=selectHtml;
            </script>
</body>
</html>